The state of the art comprises three types of planetary reduction gears for high transmission ratios:
simple planetary gearings characterized by two sun pinions with external toothings, or by two internal ring gears; PA1 planetary gearings with a number of subsequent stages of reduction, each constituted by a sun pinion, two or more planetary pinions and ring gear with internal teeth; PA1 combined planetary gearings, such as the Wolfrom type, constituted by a sun pinion which functions as driving element, two or more trains, dephased at equal angles, each of two planetary pinions, and two ring gears with internal teeth, one of which is fixed to the casing, whereas the other is rotary and functions as driven element. PA1 the tangential forces exerted by the ring gears on the respective planetary pinions are in opposite directions and tend to misalign the toothings; in the conventional version of the speed reduction unit, this effect is counteracted by bearings accomodated in a suitable cage, but without full effect, because of the clearance and of the elastic yielding of the bearings and of their mountings; PA1 the toothings of the planetary pinions of the two or more trains, which engage with the two ring gears, have to be brought into phase during manufacture in order that the load is distributed uniformly between them; PA1 the presence of two planetary pinions of different diameter on the same shaft, for each train, renders impossible, for at least one of them, the operation of grinding of the toothing by means of an abrasive wheel. All these types of epicyclic reduction gears lend themselves only with difficulty to applications in which it is necessary to reduce or eliminate completely the transmission error of the reduction unit, as due to tooth backlash and bearing clearance and understood as the possibility of angular dephasing of the driven shaft for a given position of the driving shaft. This is an increasingly frequent requirement, for example in the field of robotics, in machine tools and work centers and in measurement and control machines--all fields in which a great reduction in speed by means of a single compact reduction unit is also frequently required. PA1 there exist in fact reduction units which are comparable to planetary gearings in that they have movements of epicyclic type, but with toothings of special type and special structures, such as the so-called harmonic drive which came into being essentially for reasons of compactness but is also suitable for the requirement of reducing the transmission error due to the unit play: however, in this case too, by means of the adoption of the strictest manufacturing tolerances and high costs also for intrinsic reasons of structure; in other cases, reduction units have been adopted, also with epicyclic movements, which can be classified as cam-type reduction units and are capable of carrying out great reductions in speed: with high costs, moreover, especially if there is a requirement to reduce the play, which can be met only by means of the adoption of strict manufacturing tolerances; PA1 there exist also planetary reduction units which have toothings of the usual type, but special structures which are capable of allowing the adjustment and if necessary the elimination of the play: expensive, therefore, because of structure and bulky in that the presence of the adjustment devices impedes the utilization of the space for the transmission of the power, so that greater sizes are required in parity with power transmitted. PA1 a casing with a fixed internally toothed first ring gear; PA1 a driving shaft formed to have a coaxial sun gear; PA1 a driven shaft provided with an internal bell supporting a rotary internally toothed second ring gear; PA1 wherein said ring gears both mesh with said plurality of planetary gears, PA1 wherein the sun gear is divided into two indentical parts which mesh with the plurality of planetary gears at longitudinal ends thereof, PA1 and wherein said planetary gears are disposed so as to be free to rotate between the sun gear and the ring gears, and are formed longer than required for engagement with the ring gears, there being between said first and second ring gears a difference in the number of teeth equal to the number of the planetary gears.
The simple planetary gearings, of the first type, have very low efficiency. The planetary gearings with a number of stages, of the second type, have good efficiency but more complicated and bulky structure. The planetary gearings of the third type can be considered intermediate, in the sense that they are more compact than those of the second type, less compact than those of the first type and have average efficiency.
Of these, the Wolfrom type, which is compact and has satisfactory efficiency, has the following disadvantages however:
Some attempts to provide epicyclic reduction gears which satisfy only in part said requirement have led to very costly constructions, in that every piece is constructed with the strictest manufacturing tolerances: in particular it is necessary to eliminate the clearance of the bearings and of their mountings.
In order to meet this requirement in a more specific manner, other special types of reduction gears are also used:
This prior state of the art is capable of considerable improvements with regard to the possibility of eliminating--or in any case greatly reducing--the disadvantages listed above.
From the above, the necessity is derived of the solving of the technical problem of finding an epicyclic reduction unit such as, for example, is required in robotics, without play which results from the tolerances on the thicknesses of the teeth, on the rings of the bearings and on the mountings of the rings of the bearings, or, in any case, of very reduced play in order to eliminate the disadvantages of the Wolfrom reduction unit, without the possibility of misalignments of toothing, without any necessity for bringing the toothings into phase at the time of manufacture and such as enables the operation of grinding of the toothings of all the pinions, all this without having recourse to toothings and to structures which are not common.